glib2: don't use rb_default_rs
glib2: don't use rb_default_rs

GitHub: #125

Because rb_default_rs doesn't exist on JRuby.

Reported by vpereira. Thanks!!!

kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2